ISSN:0031-9228    

DOI:10.1063/1.2806705

出版商:AIP    

年代:1997

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.881904

出版商:AIP    

年代:1997

数据来源: AIP

ISSN:0031-9228    

DOI:10.1063/1.881905

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

What happens to a granular layer as you increase an applied shear force? In a recent experiment at Haverford College, Satoru Nasuno, Arshad Kudrolli and Jerry Gollub were able to image the particle dynamics directly. Their paper, published in the 4 August issue ofPhysical Review Letters, describes a stick‐slip behavior at low velocity gradients across the layer, with a transition to continuous motion at higher velocity gradients. Furthermore, within individual slip events, they found that the frictional force is a multivalued function of the velocity.

ISSN:0031-9228    

DOI:10.1063/1.881930

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In the summer of 1989, the European Space Agency launched the High Precision Parallax Collecting Satellite—Hipparcos, for short. The name of this first automated astrometry orbiter honors Hipparchus of Nicea, who diligently compiled a catalog of the celestial positions and brightnesses of more than a thousand stars in the second century BC.

ISSN:0031-9228    

DOI:10.1063/1.881931

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The standard way to build a laser is to produce a population inversion between an upper and a lower energy level. But laser gain is proportional not only to the population difference between two states but also to the oscillator strength, which depends on both the energy difference between the upper and lower states and the probability of a transition between them. By focusing on these often‐overlooked factors, a group at Bell Laboratories, Lucent Technologies, has made a semiconductor infrared laser with a unique property: It is electrically tunable over a 6&percent; range.

ISSN:0031-9228    

DOI:10.1063/1.881932

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

What's remarkable about this radio image of quasar1156+295(shown in false‐color form) is how it was obtained—using a radio telescope in space.

ISSN:0031-9228    

DOI:10.1063/1.881906

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

In November 1945, three months after the end of World War II, a narrow majority of the members of the Royal Swedish Academy of Sciences decided to award the 1944 Nobel Prize in Chemistry to Otto Hahn for the discovery of nuclear fission. The award was and still remains controversial, primarily because Hahn's Berlin colleagues, the chemist Fritz Strassmann and the physicist Lise Meitner, were not included. Probably, Strassmann was ignored because he was not a senior scientist. Meitner's exclusion, however, points to other flaws in the decision process, and to four factors in particular: the difficulty of evaluating an interdisciplinary discovery, a lack of expertise in theoretical physics, Sweden's scientific and political isolation during the war, and a general failure of the evaluation committees to appreciate the extent to which German persecution of Jews skewed the published scientific record.

ISSN:0031-9228    

DOI:10.1063/1.881933

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

The amplitude and phase of all light fields are subject to fluctuations. Much of this irregularity arises from random and uncontrolled changes that occur in any light source and lead to random changes in the light wave's frequency and amplitude. Most of these factors can in principle be eliminated by a careful design of the light source. Even if all of these defects in the source are removed, however, light fields are still subject to fluctuations arising from the laws of quantum mechanics. Even in a highly stabilized laser, the resulting coherent electromagnetic field has an uncertainty in the phase and magnitude of its amplitude. All conventional sources of light fields have at least this noise level, which, in a fully quantum description, arises from vacuum fluctuations. At one time it was believed that this vacuum or coherent‐state noise could not be eliminated.

ISSN:0031-9228    

DOI:10.1063/1.881934

出版商:AIP    

年代:1997

数据来源: AIP

摘要:

With the cold war over, we Americans could justifiably think that our world is safer. As a nation, we could turn from hostile pursuits and begin to enlist private and public support for other great efforts—such as building a national, nay global, information infrastructure. Such an infrastructure, besides making our lives more productive and pleasant, might also serve as a bridge to global comity.

ISSN:0031-9228    

DOI:10.1063/1.881935

出版商:AIP    

年代:1997

数据来源: AIP